1. Field of the Invention
The present invention relates to cooling devices, and more particularly to high efficiency cooling devices applied to compact three-dimensional electronic and computation systems.
2. Art Background
In recent years, electronic systems generally, and computers in particular, have become physical smaller, while at the same time becoming computationally more powerful. Typically, powerful computers have many high speed processor and memory chips to efficiently handle large amounts of data or intensive numerical computation. The high speed processor and memory chips in turn require substantial current to operate as intended. A natural consequence of high current chips is that significant heat is produced as the chips function, which heat must be removed from the computer so that the chips function properly and the physical integrity of the computer is not compromised.
For low-power chips in the range 3-5 watts, forced air convection cooling methods may be effectively employed, wherein ambient air is passed over circuit elements and modules. Frequently, heatsinks in the nature of copper or aluminum are attached to the bottoms of chips or circuit boards to first draw heat away from the chip, and then transfer the heat into the convective air stream. A representative example of a convectively cooled compact computer arrangement is disclosed in U.S. patent application Ser. No. 07/553,521 entitled "Three Dimensional Packaging Arrangement for Computer Systems and the Like", filed Jul. 13, 1990.
Conversely, for high-powered applications, for example chips consuming from 50-100 watts, closed or open loop liquid cooling systems are well known and frequently used, wherein a working fluid is passed between circuit elements and subsequently circulated to a remote heat exchanger wherein the absorbed heat is removed, after which the fluid is recirculated back to the circuit modules of the computer. Exemplary techniques for liquid cooling of high-powered compact computer systems are disclosed in the prior filed U.S. patent applications; Ser. No. 07/553,541 entitled "Apparatus for Cooling Compact Arrays of Electronic Circuitry", filed Jul. 15, 1990, and U.S. Pat. No. 5,053,856 to Davidson entitled "Apparatus for Providing Electrical Conduits in Compact Arrays of Electronic Circuitry Utilizing Cooling Devices". Lapinsky, U.S. Pat. No. 5,005,640 discloses a thin multipassage liquid cooling arrangement producing surface temperatures isothermal to within 1.degree. C. However, the arrangement taught by Lapinsky depends upon recirculating a liquid coolant through the passage to cool each cooler, and does not contemplate use of modular sealed working fluid heatpipes to avoid the added complexity of liquid recirculation. Sutrina, U.S. Pat. No. 4,631,573 discloses a stacking interposed arrangement of liquid cooled heat sinks providing high thermal efficiency. Sutrina is principally directed to providing cooling for stacked electrical components, e.g., power transistors, while permitting such components to operate at different electrical potentials, the arrangement of Sutrina providing electrical insulation between stacked components. However, the disclosure of Sutrina is dependent upon external liquid recirculation facility, and does not encompass detachable cooling/electrical modules. For very high-powered computer chips, recirculated liquid cooling, and even cryogenic cooling, are the only effective methods for removing heat from the computer.
For intermediate-power computer chips in the range 5-50 watts, it is feasible and even desirable to employ cooling methods which do not require liquid cooling with its accompanying complexity and bulk. Prior art methods for removing moderate amounts of heat include attaching heatsinks to heatproducing circuit elements, wherein the heatsinks may include sealed liquids to increase the thermal mass of the heatsink. More recently, heatpipes have been developed wherein a high vapor pressure liquid is sealed in a thin-walled thermally conductive element. One surface of the heatpipe element draws heat away from the surface contacting the heat producing body, e.g., a circuit element, and transfers it to the sealed working fluid. The working fluid is caused to locally boil, the vapor rapidly moving away from the heated region through passages in the heatpipe to cooler regions of the heatpipe. Capillary action in an applied wick returns the condensed working fluid to the region heated. A fundamental feature of a heatpipe is that the entire external surface of the heatpipe contacting a particular heat producing body can be maintained to within 1.degree. C. for reasonable power densities. The isothermal characteristic of heatpipes is of particular interest to solid state instrumentation or computation electronics, where device junction temperatures must be maintained within relatively narrow temperature ranges, and wherein temperature differences between chips on one circuit module must not vary significantly.
Although heatpipes have been previously used to cool computer and electronic systems, no previous methods or apparatus have effectively addressed cooling requirements of extremely compact general purpose computer systems, for example, a computer measuring 4.times.4.times.1 inches, and wherein circuit boards are spaced as little as 0.1 inch apart. Three-dimensional stacked electronic systems have been developed in connection with military electronic applications. For example, combat instrumentation systems or computation systems which must operate either remotely or autonomously and within a compact space have been constructed, wherein multiple circuit boards or modules are layered upon one another, separated by a material having a high heat capacity and heat of fusion. For example, in missle guidance computers, multiple electronic modules are packed together with paraffin, typically requiring 250 joules per gram of energy to melt, thus absorbing a significant amount of heat prior to the onset of deformation and melting. Such paraffin-packed computer systems are intended for extremely short lifetimes, and in military systems may not exceed five minutes. Although limited lifetime computers are appropriate for missile guidance systems which are destroyed upon impact, a five minute lifetime is essentially worthless in industrial or commercial applications.
As will be described in the following paragraphs, the present invention discloses methods and apparatus for effectively cooling a compact assembly of multiple chip electronic circuit modules containing moderate power chips without need for liquid cooling elements or interconnects. Moreover, the present invention permits integrated heatpipe/electronic circuit modules to be vertically stacked in any order consistent with the particular electronic system's logic scheme, and which permits heat to be removed from either the top or bottom of the resulting stack at the user's convenience.